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I would check Postmark's terms of service to see if you're allowed to do that. Our application uses a custom invite feature built on Zend Framework so we used Zend_Mail to build the emails including headers and then send through our SMTP server using the from field as our users email address which they registered with.

Your best bet as far as spam filters and outlooks junk filters are to test various emails, especially if you are using multi type/part emails with text and HTML versions.

Check this WebApps question. They reference an email testing service so it doesn't end up in spam. Also I found this Gmail blog entry, but the page won't load for some reason. Still, might be useful if it'll work for you guys.
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ChristopherMay 21 '12 at 4:22

Gmail actually does a great job of handling incoming emails when the From address is set from a webapp. For me it's always been Outlook which gave me the trouble of sending valid emails to the junk folder.
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AnagioMay 21 '12 at 5:40

I hate to inject personal opinion, but from my experience working with products made from a certain... 'company'... it always seems to have problems somewhere. Here's the MSDN article on the Outlook filter.
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ChristopherMay 21 '12 at 5:52